1. Field of the Invention
The present invention is related to thermoplastic molding compositions containing polyphenylene ether, a polyamide and an aspartic acid derivative.
2. Discussion of the Background
Polyphenylene ethers (PPE) are industrial high-performance thermoplastics with high melt viscosities and softening points. They are suitable for numerous industrial applications in which stability at high temperatures is important (See U.S. Pat. No. 3,306,874, U.S. Pat. No. 3,306,875, U.S. Pat. No. 3,257,357, and U.S. Pat. No. 3,257,358). Certain properties of polyphenylene ethers are also undesirable for industrial applications. For example, shaped parts made of polyphenylene ethers are brittle because of poor impact strength.
The high melt viscosities, which lead to difficulties in processing, are also a drawback. Further, polyphenylene ethers tend to be unstable and to discolor at high temperatures.
Polyphenylene ethers are soluble or swell very severely in many organic solvents. The result of this property is that they are unsuitable for applications in which they come into contact with such solvents.
It is well known that the properties of polyphenylene ether resins can be improved by blending with other polymers. Thus, for example, blends of polyphenylene ethers with high-impact strength polystyrenes have attained rather great industrial importance (See German Patents 21 19 301 and 22 11 005). These blends can be processed into molded parts with adequate toughness, but they have the drawback that their thermal stability decreases with increasing polystyrene content. These blends also have unsatisfactory solvent resistance.
Blends of polyphenylene ethers with polyamides show good flow properties and good solvent resistance (DE-OS 16 94 290, JP-A-78/47 390), but as a rule brittle products are obtained, since the two components are only poorly dispersible in one another. Aromatic polyamides such as those used in EP-OS 0 131 445, for example, also process poorly with polyphenylene ethers. Better compatibility of the two phases is achieved by functionalizing the polyphenylene ether, for example with maleic anhydride in the presence of sources of radicals (JP-OS 74/66 452). The use of radical formers, however, can lead to undesirable and uncontrolled partial gelling of the PPE phase.
It has therefore been proposed to increase the compatibility of the two polymers by adding a sufficient amount of aplasticizer, for example of an organic phosphate (EP-OS 0 129 825) or of a diamide (EP-OS 0 115 218). Such methods are unsatisfactory, since the improved compatibility has to be obtained at the expense of reduced thermal stability.
Molding compositions to which have been added copolymers of styrene and unsaturated acid derivatives have the same drawback (EP-OS 0 046 040).
The object of European Patent 0 024 120 is resin compositions that consist of a polyphenylene ether, a polyamide, a third component, and optionally, high molecular weight rubber polymers. A liquid diene polymer, an epoxide, or a compound with a double or triple bond and a functional group (such as an acid, anhydride, ester, amino group, or alcohol group) can be used as the third component. However, the toughness of the resin compositions obtained is inadequate for many applications. Added to this is the fact that most compatibility promoters such as maleic anhydride, are toxic and readily volatile. This may cause great problems for the operator during processing.
A process is described in JP-OS 74/86 653 in which 100 parts of polyphenylene ether is grafted with 10 to 300 parts of a mixture of 50 to 99% styrene and 50 to 1% unsaturated carboxylic acids or their anhydrides, by a radical method. The product is blended with polyamides. The molding compositions obtained do have very good mechanical properties, but the thermal stability decreases sharply because of the substantial fraction of polystyrene.
WO 85/05 372 describes the functionalizing of polyphenylene ethers by melting with hydroxypolycarboxylic acids or their derivatives. After blending with polyamides molding compositions are obtained whose elongations at failure and notched impact strengths indicate unsatisfactory bonding at the phase boundaries. The use of strong organic acids and the presence of water, which is formed during the functionalization, lead to the expectation of serious corrosion problems in continuous operation.
Similarly to this, in WO 86/02 086, a polyphenylene ether is functionalized by reaction with trimellitic anhydride acid chloride in solution. Blends with polyamides seem to show better property trends than in the previous case, but this process has the great drawback that either hydrogen chloride is formed or the polyphenylene ether solution contains substantial amounts of chlorides, if a tertiary amine is added, which have to be completely removed before blending because of their corrosive action on machinery and molds. Direct isolation of the polyphenylene ether by removing the solvent, which is economically superior to precipitation with methanol, is impossible in this case.